The present invention is directed to the detection of coherent signals, in general, and more particularly to a system for and a method of detecting a coherent signal of unknown frequency from an incoming electrical signal using wavelet functions.
An accurate and reliable detection of a coherent signal from an incoming signal with a low signal-to-noise ratio (SNR) is of paramount importance to accurate measurements in the signal processing for such applications as laser Doppler, phase Doppler, communications and burst radar, for example. For laser Doppler applications, a coherent burst of Doppler-shifted return signal occurs intermittently and for a brief duration for each associated parameter measurement. Of primary interest in the coherent signal is the Doppler frequency which is proportional to the parameter being measured and can range over several orders of magnitude approaching one gigahertz, for example. Existing methods and apparatus for real time coherent burst detection, also known as triggering, systems are disclosed in the patent literature.
One such patent, U.S. Pat. No. 4,973,969 entitled xe2x80x9cCoherent Frequency Burst Detector Apparatus and Methodxe2x80x9d, and assigned to TSI Incorporated, is directed to a burst detector method and apparatus for a laser Doppler Velocimeter (LDV) system based upon an autocorrelation technique. The TSI method includes correlation coefficients corresponding to frequencies that span exponentially the desired range of Doppler-shifted frequencies. But autocorrelation in general has limited ability to measure signals with low signal-to-noise ratios and requires high computational resources which result in a relatively low frequency response. Another patent, U.S. Pat. No. 5,289,391, entitled xe2x80x9cMethod and Apparatus for Optimum Signal Burst Detectionxe2x80x9d and assigned to Aerometrics, Inc. is directed to an LDV burst detector using the Discrete Fourier transformation (DFT). Although the DFT method of Aerometrics has a better response to low SNR, the DFT coefficients are spaced linearly in frequency and require more coefficients (i.e., more circuitry) to cover the same Doppler-shifted frequency range as that of the autocorrelation techniques.
The present invention improves over the drawbacks of both the autocorrelation and the DFT methods. For example, it has a SNR response comparable to the DFT method, but better than the autocorrelation method as described above. It also requires fewer calculations than both the autocorrelations and the DFT approaches for each respective coefficient which results in a higher frequency response. In addition, the present invention is well suited for applications requiring broad band frequency response and actually uses fewer coefficients than both the autocorrelations and DFT methods to cover the same Doppler-shifted frequency range. These improvements reduce the size and cost of any signal processing circuitry with no tradeoff in performance.
In accordance with the present invention, a method of detecting a coherent signal of unknown frequency from an incoming electrical signal comprising a multiplicity of frequencies comprises the steps of generating a time sequence of sample data signals from the incoming electrical signal, decomposing the sample data signals of the time sequence into a plurality of frequency range signals using wavelet functions, and detecting the coherent signal in the time sequence based on the frequency range signals.
Further, in accordance with the present invention, a system for detecting a coherent signal of unknown frequency from an incoming electrical signal comprising a multiplicity of frequencies comprises means for generating a time sequence of sampled data signals from the incoming electrical signal, means for processing the sampled data signals of the time sequence into a plurality of frequency range signals using wavelet functions, and means for detecting the coherent signal in the time sequence based on the frequency range signals.
Still further, in accordance with the present invention, a method of processing, in a programmed gate array, a time sequence of data samples of an electrical signal having a predetermined pass band frequency range for detecting a coherent signal therefrom comprises the steps of decomposing the data samples of the electrical signal into a plurality of frequency range signals within the predetermined pass band frequency range using wavelet functions, and detecting the coherent signal in the time sequence based on the frequency range signals. In one embodiment, the data samples are decomposed using wavelet packet functions. In another embodiment, the data samples are decomposed using Haar scaling and wavelet functions to form smoothed and detailed components of the frequency range signals. In yet another embodiment, the data samples are decomposed using a multi-resolution analysis method.